The use of spectrometry in analytical laboratories for measuring physical and analytical properties of materials is a well established art. Raman spectrometry is one such technique that can provide qualitative and quantitative information about composition and/or molecular structure of chemical substances. When incident radiation interacts with matter it may undergo a process called scattering. Scattered radiation may be elastic, in which the incident wavelength is unchanged in the scattered radiation, or inelastic, in which the scattered radiation has different wavelengths than the incident radiation. In one form of inelastic radiation scatter, referred to as Raman scattering, incident photons are scattered with either a gain or loss of energy. The energy difference between the scattered and incident radiation is commonly referred to as the Raman shift. The resultant Raman shift spectrum provides the energy of various molecular vibrational motions and conveys chemical and molecular information regarding the matter studied.
The Raman scattering effect is extremely weak; typically a few Raman scattered photons exist among millions of elastically scattered photons. This small Raman signal amongst the large elastically scattered signal places severe demands on the instrumental design of any spectrometer used to collect useful Raman spectra.
Numerous radiation sources are capable of generating Raman scatter from a material. For analytical measurement, these sources need to emit monochromatic radiation of high intensity. In this regard, lasers are well suited radiation sources. U.S. Pat. No. 3,556,659, the disclosures of which are incorporated herein by reference, describes a Raman spectrometer in which a sample contained in a tube is irradiated by radiation from a laser along the axis of the tube.
There are various classes of laser radiation sources, including: gas lasers, such as helium-neon, nitrogen, argon ion and krypton ion; solid state lasers, such as ruby lasers and Nd:YAG (neodymium:yttrium-aluminum-garnet) lasers; dye lasers; chemical lasers; and solid state lasers, such as single mode and multi-mode diode lasers.
Of these, gas lasers are generally accepted as especially suitable for dispersive Raman spectrometry because of their high degree of wavelength stability. Unfortunately, they are either expensive and require extensive maintenance, or they have low output power. The use of semiconductor diode lasers in Raman spectrometry, which can provide large power output in a compact, rugged device but which may exhibit inherent instabilities in their output properties, are described in Wang and McCreery, Anal. Chem., 1990, Vol. 62, pp. 2647-2651, the disclosures of which are incorporated herein by reference.
Because the Raman scattering process relates to a shift from an incident wavelength, different lasers provide spectra in different wavelength regions. However, the Raman shift spectra in those regions are similar, and essentially the same structural information can be obtained through the use of different incident laser wavelengths.
Fluorescence is a process by which absorbed radiation induces broad emission, characteristic of the molecular structure. The induced fluorescence signal, if observed, is typically many orders of magnitude larger than the Raman signal and in some cases completely masks the Raman shift spectrum. Thus, it is desirable to select an incident wavelength that minimizes fluorescence emission processes.
A well known method to reduce fluorescence background problems is to use lasers which generate red or near infrared radiation, with wavelengths from about 660 nanometers to 1100 nanometers, as described in D. B. Chase, J. Am. Chem. Soc., 1986, Vol. 108, pp. 7485-7488, the disclosures of which are incorporated herein by reference. Such a method is useful because the fluorescence emission profile is independent of incident wavelength and the Raman process is a shift from the incident wavelength. Typical radiation sources operating in this region include krypton ion gas lasers, single mode diode lasers, multi-mode diode lasers, and Nd:YAG lasers.
The large ratio of elastically to Raman scattered photons requires an efficient method of photon separation. Traditionally, this has been accomplished with double or triple spectrograph systems, constructed with two or three dispersive elements, respectively. Other radiation filtering devices can sufficiently reject the elastically scattered photons to permit the use of smaller, more efficient single dispersive element spectrograph devices; for example, holographic Bragg diffraction filters are described in Carrabba et al., Appl. Spec., 1990, Vol. 44, pp. 1558-1561, the disclosures of which are incorporated herein by reference.
The detector element is critical to the performance of the Raman instrument and must be capable of discerning extremely low levels of radiation. Traditional scanning monochromator systems have used photomultiplier tubes that are capable of observing low photon signals. More recent instruments employ array detectors such as photodiode arrays (PDA) or charge coupled devices (CCD). Array detectors consist of multiple optical elements that can simultaneously observe a region of the spectrum up to the entire Raman spectrum. CCD detectors are multi-dimensional and able to observe multiple Raman spectra at more than one wavelength simultaneously.
The previously mentioned paper by Wang and McCreery describes the use of a charge coupled device together with a near-infrared diode laser in a Raman spectrometer of high sensitivity. Also, Newman et al., Appl. Spec., 1992, Vol. 46, pp. 262-265, the disclosures of which are incorporated herein by reference, describes the use of a CCD and diode laser with a flat field imaging spectrograph provided with a fiber optic interface with the sample.
Raman spectrometry instrumentation that combines a single dispersive grating spectrograph with a CCD detector, single-mode diode laser, fiber optic cables, a fiber optic probe, and a suitable computer may be able to accomplish in seconds what used to take minutes to hours with traditional instrumentation. However, mechanical stability of the spectrograph and detector system and other optical interfaces as well as the diode laser instabilities cause severe limitations to the ultimate quantitative capability.
Fourier Transform (FT) Raman spectrometry has been proposed for quantitative chemical analysis. However, due to instrumental variations, the analysis is generally limited to, at best, about one percent reproducibility, as described in Seasholtz et al., Appl. Spec., 1989, Vol. 43, pp. 1067-1072, and in Smith and Walder, "Quantitative Analysis Using FT-Raman Spectroscopy," Nicolet Instrument Corporation technical publication AN-9145, 1991. This level of uncertainty is inadequate for many quantitative applications.